1. Field of the Invention
The present invention relates to a micro surface mount coil unit suitable for electronic equipment especially required to be small and thin, such as cellular phones, digital cameras, notebook type personal computers, and mobile computing devices including electronic notebooks.
2. Description of the Prior Art
In recent years, with the widespread use of electronic equipment etc. especially required to be micro, such as cellular phones, a coil unit used in the electronic equipment has been required to be significantly small in size.
As a coil unit capable of being significantly miniaturized, a surface mount coil unit attached to a circuit board, for example, as shown in FIGS. 6A and 6B, is known.
A coil unit 210 shown in FIGS. 6A and 6B has a drum-shaped core 211 in which flange parts 213 (upper flange part) and 214 (lower flange part) are extendedly provided at both ends of a coil form part (not shown in FIGS. 6A and 6B) around which a coil wire is wound and a lead frame 212 which is bonded to the axially outside surface of one flange part 214.
The lead frame 212 is formed by a metal sheet, and consists of left and right lead frame parts 212A and 212B bonded in a pair to the lower flange part 214 of the drum-shaped core 211 as shown in FIG. 6B. Each of the lead frame parts 212A and 212B has a pair of coil wire binding terminals 216 to which the end portion of coil wire is bound, and also has an external connecting terminal 215 soldered to the wiring pattern (electrode land) of a circuit board to which this coil unit 210 is attached.
Generally, the core 211 of the above-described coil unit 210 is formed of a breakable sintered material such as ferrite, so that the shock resistance is raised by bondingly fixing the lead frame 212 to the flange part on the circuit board side so as to cover most of the outside surface of this flange part. In this configuration, however, the width of bottom surface of the lead frame 212 is almost equal to or larger than the width of the lower flange part 214, thus being considerably larger than the width of circuit board pattern. As a result, when the bottom surface of the lead frame 212 is soldered onto the circuit board pattern to which cream solder is applied, the lead frame slips on the solder melted by heating (reflow solder), so that the coil unit turns or shifts laterally with respect to the circuit board pattern, resulting in a fear that the coil unit is attached to a position different from the designed position.
Also, the molten solder moves and spreads on the outside surface of the lead frame, so that the quantity of solder actually contributing to the connection with the pattern decreases, which may result in poor soldering.
Further, there arises a need for increasing the width of the circuit board pattern (land size), which poses a problem in that it is difficult for the coil unit to be mounted with high density according to the decrease in assembled component size.
Thereupon, in the prior art shown in FIGS. 6A and 6B, a part of the bottom surface of each of the left and right lead frame parts 212A and 212B (having almost the same width as that of circuit board pattern) is protruded to the pattern connection side to form the terminal part 215 so that this terminal part 215 is soldered to the circuit board pattern. Thereby, the widths of two elements being soldered are made almost the same, so that the above-described positional shift of coil unit and poor soldering can be avoided, and the packaging density of circuit board can be improved. Moreover, the shock resistance of coil unit can be increased.
A configuration similar to the above-described configuration is also described in, for example, Japanese Unexamined Patent Publication No. 11(1999)-283840.
In such a coil unit, a decrease in direct current resistance (DCR) value of coil wire is important for improvement in direct current superposition properties.
However, as described above, the coil unit that is used for electronic equipment especially that which is required to be small and thin must be short, and the axial length of drum-shaped core is required to be very small, being about 0.5 to 1.0 mm, for example.
Therefore, in such a short and small coil unit, it is necessary that the largest possible space for coil winding portion of the coil form be secured, and thereby the wire diameter of coil can be increased to decrease the DCR value.
Considering such circumstances, in the above-described prior art, the terminal part 215 is protruded to the pattern connection side, and hence the space for the coil winding portion of the coil form becomes short accordingly, so that there is formed a state of being inverse to the improvement in direct current superposition properties (decrease in DCR value).
The amount of protrusion of the terminal part 215 is as small as, for example, about 0.1 mm. However, if the winding width of the coil form is, for example, about 0.3 mm, the coil wire diameter becomes ¾ times as compared with the case where the terminal part 215 is not protruded, so that the DCR value undesirably becomes 16/9 times.